It's not an answer to the question, per se, but one witness at the trial of female pirates Anne Bonny and Mary Read "... knew them to be women by the largeness of their breasts"

Also, one of my favourite paintings is Nathaniel Bacon's "Cookmaid with vegetables", which features particularly juicy melons, as well as a pair of fairly large breasts from 1620-25.

I don't know how early breasts were measured and whether it would be possible to draw any quantitive data anyway, but from the above I think it's fair to say that bigger breasts have been around for centuries.

(I can't believe I managed to type all of that out without giggling like a schoolboy once).

You might also try over at /r/askanthropology or maybe /r/askscience, as this question likely deals more with evolution/sexual selection than history.

Edit: This thread in askscience has a few resources. It doesn't answer your question on a historical timescale, but it appears that the consensus is that human females have large breasts relative to other mammals, so it is reasonable to assume that the trait was selected for at some point (though it doesn't always work that way). It does not appear that there is a consensus as to why the trait was selected (if it was at all).

Exposure to chemical pseudoestrogens have increased over the 20th century in developed countries. It has been theorized in modern times to cause earlier onset puberty in girls (and to a lesser extent, delayed onset in boys.)

Also, fat (stored in adipose tissue in the human body) compromises the majority of breasts. Any population (regardless of time period) that experiences a steady increase in access to food will steadily produce a population with increased adipose storage. This will in turn increase breast size in a linear fashion.

Edit Sources:

Plastics, rise of usage starting in the 40's to today, along with ways they get in the human body. A published overview

Most plastic products release estrogenic chemicals

Use of plastics, notably BPA, in regards to early puberty of girls

Composition of the breast tissue as fat 69.9+/-22.9%

They are all medical journal articles but current findings can be retroactively applied to explain historical phenomenon.

Perhaps the Historians present would be better able to ask about relative fashionableness of large/small breasts and different body types in general?

I have the impression, for instance, that during the baroque period, a chunkier body-type was considered more beautiful, based on paintings of the day.

Even in the 1950s, certain song lyrics make me wonder if that was still the case. In Bony Moronie, the singer appears to be saying "she's all skin and bone but I love her anyway", while in Reet Petite, the fact that "she really fills her clothes / from head to toe" suggests that this is considered a good thing.

It's hard to say because any contemporary account is usually biased by the standards of beauty of the time. I would recommend the book History of the Breast by Marilyn Yalom, who describes (among many other things) the historical European stereotype of poor women with large breasts vs. aristocratic women with smaller more fashionable (about b-cup) breasts. The implication was that rich women hired the poorer women as wet nurses.

This is reflected in paintings, where a women working in a kitchen or in an agrarian scene will have larger breasts, but an aristocratic woman or goddess/historical figure will be depicted with smaller breasts.

A note to all readers: There has been a high amount of speculation in this thread. If you're unable to provide anything beyond conjecture, and you can't give any source more reliable than a Wikipedia link, then please refrain from posting. If you can't answer the question then you don't need to contribute.

Some degree of informed speculation is going to occur but I doubt you're ever going to have a clear answer. Body and Breast sizes have only really been formalised since the introduction of modern manufacturing. There was no clear identifiable 'size 12' or "14DD" prior to the 20th century (clothing 'sizes' did exist but they were even less standard than today).

Thus there is no statistical way you can determine any increase in breast size even if you could assume that the sizing was consistent and accurate (which it isnt now).

Prior to this it would have socially unacceptable to measure, quantify or even discuss this sort issue in any detail. So you're left with incidental data introduced from other sources, literature or painting really. Even then you're not going to get anything specific. Someone like Pepys may have made mention of features recognisable as big or small bosoms (although I cant recall any), but theyre not going to be in any scientifically usable form. Art will merely show you what effect it wants to achieve, busty milk maids, demure young ladies. The Venus of Hohle for example shows your that exaggerated feminine forms have a symbolic power, but tell you nothing about 'average' values.

As others have pointed out Humans are almost, if not, unique in their breast sizes in females. So whatever was selected for was probably a long time ago. I doubt its continuing unless you can provide evidence that larger women have more children than smaller ones. I suspect other aspects are in play these days. Of course its skewed today because as western women get fatter each generation their breasts are increasing in size as well.